Method for delignifying lignocellulosic raw materials

ABSTRACT

The invention relates to a method for delignifying lignocellulosic raw materials by using sulfites in the presence of an alkaline component, especially sodium hydroxide or sodium carbonate or a mixture thereof in an aqueous solution at a high temperature and high pressure. The invention is characterized in that a first partial fragment of the alkaline component is added when the aqueous solution starts to decompose and in that at least a second partial fragment of the alkaline component is added only when delignification begins.

[0001] The invention relates to a method for delignifyinglignocellulosic raw materials. Such a method is technically also knownas pulping.

[0002] Lignocellulose containing raw materials, such as wood or grassesare used for the manufacture of cellulose. In order to minimize bothenergy consumption in cellulose manufacture and the pollution of theenvironment, it is desirable to remove as much lignin as possible in thefirst process step, i.e. pulping, without degrading the cellulose toomuch. Only when delignification can be continued until only a smallresidue of lignin remains, it is possible, using reasonable amounts ofchemicals, to bleach to high grades of whiteness.

[0003] Known methods for delignifying lignocellulosic raw materialsusing sulfites as an effective lignin reducing component (sulfitepulping) are carried out in an acidic, neutral and alkaline pH ranges.The methods in neutral and alkaline pH ranges only lead to small amountsof delignification. If a quinone component is added in these methods,delignification is improved to significantly lower lignin residuepercentages, but the remaining lignin percentage is still too high toachieve bleaching to high degrees of whiteness under economicalconditions. If either pulping or bleaching is carried out underextremely severe conditions, usually not feasible on an industrialscale, acceptable results may be achieved, but the yield and especiallythe strength of the fibres are drastically reduced.

[0004] This is why, in practice, fibres made with the AS-AQ method(alkaline sulfite method with anthraquinone) and the NS-AQ method(neutral sulfite method with anthraquinone) are primarily used forunbleached or semi-bleached cellulose products. These celluloseproducts, characterized by a high lignin residue content, but withexcellent yield and good strength, are suitable, for example, for themanufacture of corrugated cardboard products.

[0005] It is therefore an object of the present invention, to provide amethod for delignifying lignocellulosic raw materials, wherein by usingsulfites as a lignin-degrading component for pulping methods in theneutral or alkaline ranges the lignin residue content may be minimized.

[0006] This object has been achieved by having sulfites in the presenceof an alkaline component, in particular sodium hydroxide or sodiumcarbonate or a mixture thereof, in aqueous solution with the applicationof high temperature and high pressure, cause extensive delignificationby adding a first portion of the alkaline component to the aqueoussolution at the beginning of the pulping process and by adding at leasta second portion of the alkaline component to the aqueous solution atthe beginning of delignification or later. A significant reduction ofthe pH value during heating is accepted quite deliberately, it is evenessential for maximizing lignin degradation.

[0007] Sodium hydroxide (NaOH) or sodium carbonate (Na₂CO₃) is primarilyused as the alkaline component, potassium or ammonium compounds,however, are also suitable.

[0008] The numerous references on sulfite pulping in neutral andalkaline ranges agree that all pulping chemicals, i.e. the sulfite, thealkaline and, if necessary, also the quinone component are added to theaqueous solution at the beginning of the pulping, i.e. before heating topulping temperature. Increasing the overall percentage of chemicals,which means adding great quantities of sodium hydroxide, usually leadsto a low, albeit stagnating at a high level, residual lignin content.The use of extreme quantities of sodium hydroxide may result in fibresbleached to a high degree of whiteness, but the fibres are severelydamaged, leading to drastic losses in viscosity, and therefore strength.Persons skilled in the art, when dealing with maximum delignification,therefore always recommend keeping alkaline content as high as possiblefrom the start. This opinion is supported by the fact that pH values aresignificantly reduced when the main delignification phase ends. It isconsidered essential to keep the level of the alkaline component as highas possible before the beginning of the pulping, in order to removeenough lignin for the wood to be decomposed into fibres.

[0009] DE 1 815 383 (to Ingruber) is particularly clear about this.Ingruber teaches to control pH values from the beginning of the pulping,and to ensure that the high alkaline pH value set at the beginning ofpulping is maintained invariable by constantly adding NaOH during theheating and also in the subsequent steps of pulping. The pulping resultsdisclosed in this reference show that while the wood mass may be pulpedwith a low residual lignin, using extreme amounts of chemicals, at a noteconomically feasible level, of 50% with absolutely dry wood mass, atthe price of low yields and extraordinary losses of strength.

[0010] As exemplary references for the prior art alkaline and neutralsulfite methods, the following publications are cited: SA patent77/3044, (1977); U.S. Pat. No. 4,213,821; JP 112903; EP 0 205 778;Gierer, I., “Über den chemischen Verlauf der Neutralsulfitkochung” (“Onthe chemical profile of neutral sulfite cooking”), Das Papier 22, Volume10A, from p. 649 (1968); Gellerstedt, G. “The reaction of lignin duringsulfite pulping” Svenrsk Papperstidning 79, from p. 537 (1976); Gierer,I., Lindeberg, O. and Noren, I. “Alkaline delignification in thepresence of anthraquinone/anthrahydroquinone”, Holzforschung 33, pp213-214 (1979); Ojanen, E., Tuppala, Virkola, N. E. “Neutral SulphiteAnthraquinone (NS-AQ) Cooking of Pine and Birch Wood Chips”, Paperi jaPuu 64, from p. 453 (1983); Virkola, N. E., Pusa, R., Kettunen, J.“Neutral Sulphite AQ Pulping as an alternative to Kraft pulping” TAPPI64, from p. 103 (1981); Tikka, P., Tuppala, J. Virkola, N. E. “NeutralSulphite AQ pulping and bleaching of the pulps” TAPPI InternationalSulfite Pulping Conf. Proceedings, from p. 11 (1982); Raubenheimer, S.,Eggers, S. H. “Zellstoffkochung mit Sulfit und Anthrachinon” (“Cellulosecooking with sulfite and anthraquinone”), Das Papier 34, vol. 10A, fromp. V19 (1980); Ingruber, O. V., Stredal, M., Misted, J. A., “AlkalineSulphite—Anthraquinone Pulping of Eastern Canadian Woods”, Pulp & PaperMagazine of Canada 83, Vol. 12, from p. 79 (1981); ingrmlber, O. V.,“Alkaline SuIlphite Anthraquinone Pulping”, TAPPI International PulpingConference, Hollywood, Proc. Vol. II, from p. 461, (1985); Cameron, D.W., Jessupa, B., Nelson, P. F., Raverty, W. D., Samuel, E., Vanterhoeck,N., “The response of pines and eucalyptus to NSSC-AQ-Pulping” Ekman Days1981, Stockholm, Vol. II, from p. 64; Suckling, I. D., “The role ofanthraquinone in sulphite-anthraquinone pulping”, TAPPI Wood and PulpingChemistry Symposium, Proceedings, from p. 503 (1989); U.S. Pat. No.5,409,570 describes adding of NaOH before an oxygen stage which iscarried out subsequent to the chemical pulping.

[0011] It is all the more surprising therefore that adding alkalinecomponents in at least two portions at a time interval (alkalisplitting) results in delignification can be continued until very lowresidual lignin is achieved, wherein the yields remains stable, or mayeven be increased, and losses in strength may be avoided. As anindicator for the condition of the cellulose, the viscosity also showsimproved values in spite of the reduced residual lignin. The at leastone second portion of the alkaline component should not be added beforethe beginning of delignification. This process starts as early as a fewminutes after the beginning of pulping, during the heating of thelignocellulosic raw material and the aqueous solution containing thepulping chemicals. The advantageous effect of alkali splitting is morenoticeable the later the at least one second portion of the alkalicomponent is added, where there is a broad optimum range for the maximumpulping temperature. at least one second portion of the alkali componentis added, where there is a broad optimum range for the maximum pulpingtemperature.

[0012] Contrary to previous knowledge of persons skilled in the art, ithas turned out to be advantageous to accept a reduction of the pH valuewhile heating to the maximum pulping temperature. For Example with aninitial pH value of 13.0 set at the beginning of pulping, the pH valueis reduced depending on the alkaline component added at the beginning ofthe pulping process to values of pH 8.0 (12.5 wt. % of the overallamount of the alkaline component added at the beginning of the pulpingprocess) to pH 10.75 (50 wt. % of the overall amount of the alkalinecomponent added at the beginning of the pulping process). However, if100 wt. % of the alkaline component is added already at the beginning ofthe pulping process, pH values will only fall.

[0013] According to the teachings of the present invention it has provenadvantageous for the at least one second portion of the alkalinecomponent to be added after the pH value of the aqueous solution hasfallen during the heating process, at least by an amount of pH 0.3,preferably by an amount of pH 0.5, more advantageously by an amount ofpH 1.0, most advantageously by an amount of at least pH 1.5, each timewith reference to the initial pH value of the pulp. While advantageouseffects with respect to cellulose characteristics and yields becomesufficiently clear when the at least one second portion of the alkalinecomponent is added at a relative early stage, i.e. at a pH valuedifference of at least 0.3 with reference to the initial pH value, thepositive effects with respect to the cellulose characteristics andyields are greater if the at least one second portion of the alkalinecomponent is only added after the pH value of the aqueous solution hasfallen by an amount of at least pH 1.0, more advantageously by at leastpH 1.5, vis-à-vis the initial pH value.

[0014] It has proven advantageous for the addition of the at least onesecond portion of the alkaline component to be carried out only after atleast 30% of the portion of the alkali originally used is used up, i.e.is no longer detectable in the aqueous solution containing the chemicalsused for pulping. Another improvement of the pulping result, inparticular lignin decomposition, can be expected if before the additionof the at least one second portion of the alkaline component, a minimumof 90%, preferably 95%, of the alkali added with the first portion, areused up.

[0015] Delaying the addition of the at least one second portion by aslittle as 10 minutes after the beginning of the pulping process alreadyimproves the fibre characteristics and yields of the lignocellulosic rawmaterial. A further time delay between the beginning of the pulpingprocess accompanied by the addition of the first portion of the alkalinecomponent, and the addition of the at least one second portion of thealkaline component shows further significantly improved cellulosecharacteristics and good yields within a broad time range.Advantageously, the at least one second portion of the alkalinecomponent is added no sooner than 30 minutes, more advantageously notbefore than 60 minutes, most advantageously no sooner than 90 minutesafter the beginning of the heating.

[0016] The addition of the at least one second portion of the alkalinecomponent after a temperature of at least 75° C. has been reached byheating the aqueous solution containing the pulping chemicals and thelignocellulosic raw material causes an improvement of the fibrecharacteristics and the yields as compared with a pulping process, whichis carried out identically, yet without alkali splitting. Significantimprovements of the cellulose quality and the yields are achieved byadding the at least one second portion of the alkaline component after atemperature has been reached of 110° C. or higher, more advantageouslyof 140° C. or higher, most advantageously of 175° C. or higher.

[0017] The lignocellulosic raw material and the aqueous solutioncontaining the sulfite and the alkaline and, where applicable, thequinone components, i.e. the aqueous solution containing the pulpingchemicals, is collectively heated to the maximum pulping temperature. Ithas been found to be particularly effective for the at least one secondportion of the alkaline component to be added only after the maximumpulping temperature has been attained. If the addition of the at leastone second portion of the alkaline component is triggered, for example,by a process control, it is conceivable that the addition of the atleast one second portion is activated, for example, when a minimumtemperature of 150° C. is reached, or when a predetermined situationdepending on the raw material and other pulping parameters used, occurs,such as pH value or time.

[0018] Cellulose with good strength and low residual lignin is obtainedwhen pulping is carried out for a duration of 90 minutes or longer,preferably 120 minutes or longer, advantageously 150 minutes or more or,most advantageously 360 minutes or longer. The overall duration of thepulping process is relatively short, lasting only between 90 and 360minutes, which is due to the fact that in the method according to theinvention, delignification occurs already to a considerable degreeduring the heating phase by a reduction of the pH value and that furtherdelignification, after adding the at least one second alkaline portion,is well prepared.

[0019] A preferred embodiment of the method according to the presentinvention provides for the pulping of the lignocellulosic raw materialin the aqueous solution containing the sulfite and the alkalinecomponent and, if applicable, the quinone component, to be carried outwith a pulping duration of at least 30 minutes, preferably between 60and 360 minutes, more advantageously between 120 minutes and 180minutes, at a maximum pulping temperature.

[0020] Even though the degree of delignification is increased, theduration of the pulping process at maximum temperature can be madeshort. With raw materials having low lignin content, such as annualplants or hardwoods with little lignin content, as little as 30 minutesmay be sufficient. When pulping wood chips, the duration of the pulpingprocess is preferably between 60 and 180 minutes, usually between 120and 150 minutes, at maximum temperature. If for technical reasons, arelatively low pulping temperature between 160° C. and 170° C., ischosen, for example, it may be necessary to increase the pulping time to300 minutes at maximum temperature.

[0021] The pulping process in which the alkaline component is added inat least two portions at a time interval may be carried out usingrelatively mild conditions. At a pulping temperature of as little as150° C., for example, bleachable celluloses may be obtained after 60minutes. Preferably, the maximum pulping temperature is between 160° C.and 180° C. If the lignocellulosic raw material is hard to pulp, thetemperature may be increased, wherein the economical limit is about 190°C.

[0022] In the most basic case, the first and second portions of thealkaline component can be about equal, i.e. about 50 wt. % at thebeginning of the pulping process and about 50 wt. % when the maximumpulping temperature is reached, for example. It came as a surprise thenthat adding as little as about 15 wt. % as the first portion of thealkaline component at the beginning of the pulping process and a laterdosage of 85 wt. % as the second portion of the alkaline component leadsto excellent delignification results.

[0023] According to the present invention, the effect of extensivedelignification is achieved when the first portion of the alkalinecomponent is between about 15 wt. % and about 80 wt. %, and whencorrespondingly about 85 wt. % to about 20 wt. % of the alkalinecomponent are added as a later dose of the at least one second portion.Of particular advantage is a separation between about 75 wt. % to about30 wt. % of the alkaline component at the beginning of the pulpingprocess and between about 25 wt. % and about 70 wt. % of the alkalinecomponent after the beginning of the delignification. Preferably,between about 60 wt. % and 40 wt. % are added as the first portion ofthe alkaline component and between 40 wt. % and 60 wt. % as the secondportion of the alkaline component. In particular, about 50 wt. % of thealkaline component as each of the first and second portions have provento be maximally effective for delignification while at the same timebeing mild on the cellulose fibres.

[0024] The overall percentage of chemicals, i.e. sulfite with alkalinecomponent and, if applicable, quinone or sulfide components, and, ifapplicable, the addition of alcohol, can be kept low. With raw materialshaving a low lignin content, as little as 18 wt. % or more overallpercentage of chemicals with absolutely dry wood is sufficient toachieve extensive delignification. If hard-impregnating wood with a highlignin content is to be pulped, as much as 45 wt. % overall chemicalswith absolutely dry wood must be used. Depending on the raw material,the overall percentage of chemicals can be chosen from a wide range.Good delignification results can be achieved with an overall percentageof chemicals of between about 22 wt. % and about 45 wt. %, preferablywith an overall percentage of chemicals of between about 25 wt. % andabout 35 wt. %, advantageously of between about 28 wt. % and about 32wt. %. For conifer wood, generally an overall percentage of chemicals ofbetween 22 and about 30 wt. %, preferably between about 25 and about 28wt. % with absolutely dry wood is sufficient; for hardwoods, the overallpercentage of chemicals may vary widely between about 20 and about 30wt. % depending on the kind of wood.

[0025] Regardless of the overall percentage of chemicals chosen, theratio between sulfite and the alkaline component can be widely adjusted.Since the quinone component added as needed is only used in minimalamounts, it is negligible for adjusting the ratio of sulfite to akali. Aratio of sulfite to alkali components in a range of between 80 to 20 and40 to 60 is suitable to obtain celluloses of good quality. A ratio ofsulfite to alkaline component of between 70 to 30 and 50 to 50, inparticular 60 to 40, is preferred. The splitting of the overall quantityof the pulping chemicals, i.e. sulfite and alkaline component, can beadjusted, as needed, depending on the lignocellulosic raw material andthe parameters of the pulping process chosen (temperature, duration).

[0026] While splitting the alkali into two portions is alreadysufficient to obtain excellent celluloses with a low residual lignincontent and good yields and strength characteristics, the splitting intothree, four or more portions can also achieve extensively delignifiedcelluloses with high yields and good strength results.

[0027] The invention is also directed to a cellulose, obtained by themethod for delignifying according to at least one of the precedingclaims, in particular cellulose with a residual lignin after pulping ofless than kappa number 35, preferably less than kappa number 30, morepreferably less than kappa number 25, most preferably of less than kappanumber 20. The low residual lignin ensures good bleachability. Goodbleachability is characterized by the use of small amounts of bleachingchemicals and/or small energy consumption to achieve degrees ofwhiteness above 88% ISO.

[0028] Within the scope of the present invention, a cellulose isobtained according to the above described method of delignifying with aresidual lignin content after pulping of less than kappa number 35 andan accept yield of at least 45%, preferably at least 50%, both withabsolutely dry wood, preferably a kappa number of less than 30 and anaccept yield of at least 45%, preferably at least 50%, both withabsolutely dry wood, advantageously a kappa number of less than 25 andan accept yield of at least 43%, preferably at least 46%, both withabsolutely dry wood, most advantageously a kappa number of less than 20with an accept yield of at least 43%, more advantageously at least 46%,both also with absolutely dry wood. As described above, the mildness ofpulping process can be seen in the fact that lignin is removedselectively without excessively degrading or decomposing the fibres, inparticular cellulose or hemicellulose.

[0029] First attempts involving a short chlorine-free bleaching sequence(O Q(OP) Q P) of the cellulose manufactured according to the method ofthe present invention show that a fully bleached cellulose can bemanufactured with a degree of whiteness of above 88% ISO and withstrength characteristics that are reduced by as little as 5% vis-à-visunbleached cellulose. This proves the high selectivity.of the method ofthe present invention, whereby the carbohydrate component of the rawmaterial, which in prior art pulping methods is often heavily damagedinitially and is then significantly decomposed during bleaching, remainslargely intact in the present mild pulping method.

[0030] Details of the method of the present invention are explained asan example using the tests described below.

[0031] The parameters obtained in the Examples below, such as residuallignin, degree of whiteness, viscosity and strength characteristics,were determined using the standard procedures as follows:

[0032] The viscosity was determined according to Merkblatt (Code ofPractice/CP) IV/36/61 of the Verein der Zellstoff-und Papier-Chemikerund-Ingenieure (Zellcheming) (“Association of Cellulose and PaperChemists and Engineers”). The degree of whiteness was obtained bymanufacturing test sheets according to Zellcheming CP V/19/63;measurements were taken according to SCAN C 11:75 with an elrepho 2000type photometer; the whiteness is given in percent according to ISOstandard 2470. The residual lignin (kappa number) was determinedaccording to Zellcheming CP IV/37/63. The technological characteristicsof the paper were determnined using test sheets manufactured accordingto Zellcheming CP V/8/76. Unit weight and tearing strength weredetermined according to Zellcheming CP V/11/57 and V/12/57. The tearfactor was obtained according to DIN 53 128 Elmendorf. The freeness wasmeasured according to Zellcheming CP V/3/62. The yield was calculated byweighing the raw material used and the cellulose obtained after pulping,which was dried at 105° C. to constant weight (absolutely dry). Themeasurement of the tensile, tear and burst indices was carried outaccording to TAPPI 220 sp-96.

[0033] In all of the following Examples, the indications on the overallpercentage of chemicals and the splitting of the sulfite component andthe alkaline component are calculated as NaOH.

EXAMPLE 1

[0034] Pine-wood chips were mixed with an alkaline sodium sulfitepulping solution after vaporization (30 min. with saturated vapour at105° C.) at a liquid-to-solid ratio of 4 to 1. The overall percentage ofchemicals with absolutely dry wood was 27.5 wt. %. The alkali ratio ofsodium sulfite to NaOH was adjusted to 60 to 40. In the abovepreliminary study with reference to FIG. 1 regarding the alkalinesulfite pulping with antraquinone, this ratio has proven to be a goodcompromise between maximum delignification and minimum viscosity loss.FIG. 1 shows quite clearly, however, that a wide range of mixing ratiosfor the sulfite component and the alkaline component lead to goodpulping results. The preliminary studies were carried out under thereaction conditions as outlined in Example 1, wherein, however, 100% ofthe sodium-hydroxide solution was added at the beginning of the pulpingprocess.

[0035] It was not until the “modified” tests shown in Table 1 that theNaOH amount was divided. Half of the amount of sodium hydroxide solutionwas added to the pulping solution as a first portion (50%) together withthe sodium sulfite and 0.1 wt. % anthraquinone with absolutely dry wood.The raw materials and the pulping solution was then heated for 90minutes to reach 175° C. Then the second portion of the NaOH (50%) wasadded in an aqueous solution. This increases the liquid-to-solid ratioto 5 to 1. The pine-wood chips were then pulped at 175° C. for 150minutes. Subsequently the cooker was degassed, cooled down to below 100°C., and the pulp was taken out. It is washed, the chips are ground in apulper and thus disintegrated into fibres. The fibres are sorted in aslot sorter. Then the yield, residual lignin (expressed in a kappanumber), degree of whiteness, tearing strength and bursting strengthwere analysed. The results are shown in Table 1 in the line labelled“modified”.

[0036] As a reference Example, conventional alkaline sulfite cooking wascarried out. Raw materials and test conditions corresponded precisely tothe ones of Example 1, except that 100% NaOH is added before heating.The time and temperature profile of the reference Example alsocorresponded to the time and temperature profile of Example 1. Theprocessing and analysis of the pulp was carried out in the same manneras in Example 1. The results are shown in Table 1 in the line labelled“standard”.

EXAMPLE 2

[0037] Under the same conditions as in Example 1, spruce-wood chips werepulped instead of pine-wood chips. Temperature and time profile andprocessing and analysis matched the conditions indicated for example 1.The reference pulping carried out with spruce-wood chips was carriedout, processed and analysed under the conditions indicated forexample 1. The results are shown in Table 2.

EXAMPLE 3

[0038] Spruce chips were pulped again using an alkaline sulfite solutionat a maximum temperature of 175° C. for 150 minutes. The maximumtemperature was reached after a heating-up phase of 90 minutes. Theoverall percentage of chemicals was 27.5 wt. % with absolutely dry wood,and an additional 0.1 wt. % anthraquinone. The ratio of sodium sulfiteto NaOH was 60 to 40. 25 wt. % of NaOH were added before the heatingphase as a first portion. 75 wt. % of NaOH were added in an aqueoussolution after 90 minutes when the maximum pulping temperature of 175°C. was reached. The processing and analysis of the test described inExample 3 were carried out as described in Example 1. The results ofthis test are compiled in Table 3 in the line labelled “modified”.

EXAMPLE 4

[0039] An alkaline sulfite pulping process with the addition of a firstportion before heating and the addition of a second portion after themaximum temperature of the pulp has been reached can still be improvedwith respect to delignification and selectivity by adding a low-boilingalcohol (ASAM process with split addition of the alkali component).

[0040] Spruce chips were pulped under the conditions of Example 3, wherethe aqueous pulping solution, which was provided with a dose of just 25%of all the alkali before heating, was then dosed with 10 vol. % methanolwith absolutely dry wood. The processing and analysis were carried outas described in Example 1. The results of this test are described in theline labelled “ASAM modified” in Table 3.

[0041] When comparing the results shown in Tables 1 to 3, it is evidentthat the yield is hardly reduced in spite of the significantly reducedresidue, or, in the case of the modified test of Example 2, has evenbeen stabilized. Since delignification was continued here with residuallignin which in a “standard” test would have been achievable only withmuch more severe conditions, if at all, and would have led to a drasticreduction in yield, this shows an extraordinary advantage of the methodaccording to the present invention.

[0042] The viscosities achieved are another advantage of the extremelyselective methodology of the present invention, i.e. essentiallydirected to the decomposition of lignin rather than cellulose orhemicellulose. Viscosity is an indicator for the state of the celluloseat the end of the pulping process. With the “modified” tests of thepresent invention, values above the viscosities of the “standard” testsare obtained on a regular basis. If the viscosities of the tests under“modified” conditions are set in relation to the extremely low contentof residual lignin (kappa number), it is evident how mild the effects ofthe method according to the present invention are on fibres.

[0043] The strength characteristics of the celluloses pulped in the“modified” way also have the same or improved values as compared withthe fibres manufactured according to the reference tests. Again it is tobe noted that this high level of strength is maintained at a much lowerresidual lignin content. If prior art delignification methods remainunchanged or if more severe pulping conditions are used until such lowresidual lignin values—to kappa numbers below 25—are reached, if theyare reached at all, a drastic reduction in viscosity and strength valuescan be observed, since toward the end of the pulping, not only thelignin remaining in the raw material, but also the cellulose andhemicellulose are degraded and decomposed.

[0044] Particular note should be taken of the results of the modifiedASAM pulping in Table 3, where an exceptionally low residual lignincontent is obtained with a yield considerably above 47%, with highviscosity and strength values. This cellulose therefore has the bestpossible preconditions for bleaching to high degrees of whiteness at lowpercentages of chemicals used.

[0045] For other cellulose manufactured according to the “modified”methods according to the present invention shown in Tables 1 to 3, italso applies that with the extensively reduced residual lignin contentsit is also possible to bleach to high degrees of whiteness using theusual chlorine-free processes such as oxygen, ozone or peroxidebleaching. Since the cellulose manufactured using sulfite, while showinglow delignification, already has relatively good possibilities ofdecomposable residual lignin, it may be expected that the fibresmanufactured according to the modified method according to the presentinvention are capable of being bleached with low energy consumptionwhile achieving good viscosity and strength characteristics.

EXAMPLE 5

[0046] Spruce chips were pulped in an alkaline sulfite pulping processwhere the reaction conditions matched those of Example 1, except thatanthraquinone was not added. The content of residual lignin, as shown inTable 4, had a kappa number of 92.8, which was considerably above whatwould be acceptable for further processing. This test shows that evenwhen compared with a pulping process where all of the alkali componentis added at the beginning of the pulping and a residual lignin contentwith a kappa number of 100 or more is expected, the positive effect ofsplitting the alkaline addition can be observed even under these severepulping conditions.

EXAMPLE 6

[0047] In two tests, the process temperature of 175° C. was lowered to170 and 165° C., respectively, wherein the duration of the pulpingprocess at 170° C. was extended to last 210 minutes, and at 165° C. tolast 270 minutes, while the remaining process conditions of Example 1were left unchanged.

[0048] The results are shown in Table 5. The lowering of the processtemperature still results in selective processing despite longerpulping. The residual lignin is stabilised at a low level while, at thesame time, the yield and viscosity and, associated with the higherviscosity, the strength characteristics are improved.

EXAMPLE 7

[0049] Beech wood was pulped with an overall percentage of chemicals of27.5 wt. % with absolutely dry wood at a ratio of sulfite to NaOH of 50to 50 at 150° C. The beech chips were heated together with the pulpingsolution for 90 minutes to reach a maximum pulping temperature of 150°C. 0.1 wt. % anthraquinone (AQ) was added to the pulping solution. Theliquid-to-solid ratio was 4 to 1 at the beginning of the pulpingprocess. The effect of the first portion of the alkaline component(NaOH) was studied, which was varied between 0 and 100% in steps of 12.5wt. %. When the maximum pulping temperature was reached, the secondportion of the alkaline component was added.

[0050]FIG. 2 clearly shows the reduction of the pH value during heating.This is most noticeable when the first portion of the alkaline componentis 25 wt. % or less. Table 6 shows the results of these pulpingprocesses, evaluated for the parameters of yield (accept and splitter),kappa number, viscosity, end-pH value (pH value at the end of thepulping process at maximum temperature), degree of whiteness, tearingstrength and tear factor. The tests no. 31, 32 and 39 are repetitions oftests 26 to 28.

[0051] The degradation or reduction of the characteristics of thecellulose after a reduction of the pH value during heating and pulpingat maximum temperature which would have to be expected according to theprior art (cf. Ingruber, in particular), do not in fact occur. If thechosen pulping conditions are used, it can be shown that when beech woodis pulped, the alkali splitting leads to improved yields with asimilarly low residual lignin content (kappa number) and a high degreesof whiteness, when the first portion is as high as 37.5 wt. % of NaOH.

EXAMPLE 8

[0052] Spruce wood was pulped with an overall percentage of chemicalsof, again, 27.5 wt. % with absolutely dry wood at a ratio of sulfite toNaOH of 60 to 40 at 175° C. The spruce chips were heated together withthe pulping solution for 90 minutes to reach a maximum pulpingtemperature of 175° C. 0.1 wt. % anthraquinone (AQ) was added to thepulping solution. The liquid-to-solid ratio was 4 to 1 at the beginningof the pulping process. The pulping conditions thus matched those ofExample 1.

[0053] The effects of varying the first portion of the alkalinecomponent (NaOH) between 0 and 100% in steps of 12.5 wt. % were studied.When the maximum pulping temperature was reached, the second portion ofthe alkaline component was added. FIG. 3, just like FIG. 2, clearlyshows the reduction of the pH value during heating. This is mostnoticeable for pulping spruce wood when the first portion of thealkaline component is 12.5 wt. % or less. Although the pH value isreduced minimally during the entire pulping process when 100% of thealkaline component is added from the beginning, it can be seen, thatwhen alkali splitting is used, the pH value is significantly reduced, inparticular during the heating phase; according to Ingruber, this effectis supposed to be deleterious, for extensive delignification, however,it turns out to be essential. If the first portion of the alkalinecomponent is only reduced to 75% of the entire amount, a reduction ofthe pH value by about 0.5 vis-à-vis the initial pH value can be seen.The reduction of the pH value is more noticeable if only 50% of the NaOHor less is added at the beginning of the pulping process. The pH valuefalls from about 13.1 at the beginning of the pulping process to aminimum value of about pH 8.5 during the heating phase. Once this pointhas been reached, the second portion of the alkaline component is added,resulting in an extensively delignified cellulose with high strength andhigh yields.

[0054] Table 7 shows the results of these pulping processes, evaluatedfor the parameters of yield (accept and splitter), kappa number,viscosity, end-pH value (value of pH at the end of the pulping processat maximum temperature), degree of whiteness, tearing length and tearfactor.

[0055] With the pulping conditions chosen, when spruce is pulped and afirst portion of NaOH of just 12.5% is used, alkali splitting results ina small residual lignin content (kappa number) and an improved degree ofwhiteness. In addition, the strength values are better when the alkaliis divided than when 100% of the alkali is added “from the start”. Thetear factor in particular, has good values. The overall high strengthlevel can be seen from the significantly higher viscosity values. Theend-pH value of all pulping processes does not show any variations, i.e.does not reflect the varied pH-value profile of the cooking process. Itshould be noted that all pH value measurements were carried out at roomtemperature.

[0056]FIG. 3 illustrates a pulping process in which the second portionof the NaOH was added after 90 minutes. It has been shown, however, thatthe effects measured, i.e. the advantages of the method according to thepresent invention, may already be seen in the manufactured cellulose, ifthe second portion of the alkaline component is added after a reductionin the pH value has been measured. The same applies to a minimumtemperature reached during the pulping process or during the heatingprocess: the addition of the second portion of the alkaline component ata minimum temperature of 75° C., preferably of 100° C., advantageouslyof 140° C., results in a cellulose, with a lower lignin content, betterstrength characteristics and higher yields when compared to cellulosemanufactured without alkali splitting.

EXAMPLE 9

[0057] The effect of alkali splitting is particularly noticeable in thepulping of pine. The process conditions for pulping the pine chips areexactly those as chosen in Example 8 for spruce.

[0058] Table 8 shows that when a first portion of NaOH—between 25% and50% of the entire amount—is added at the beginning of the pulpingprocess, a significantly lower level of residual lignin is obtained witha nearly unchanged yield, a high overall strength and a considerablyimproved degree of whiteness.

EXAMPLE 10

[0059] Eucalyptus wood was pulped with an overall percentage ofchemicals of 27.5 wt. %, with a ratio of sulfite to alkali of 50 to 50at a maximum pulping temperature of 165° C. Maximum pulping temperaturewas reached in 90 minutes. A first pulping process without alkalisplitting (so-called standard cooking) and a second pulping processwhere a first portion of NaOH of 50 wt. % at the beginning of thepulping process and a second portion of 50 wt. % was added afterreaching the maximum pulping temperature of 165° C. after 90 minuteswere carried out in parallel. The results of these cooking processesshow that the standard cooking process results in cellulose with a kappanumber of 16.8 while the alkali splitting leads to a kappa number of14.8. The degree of whiteness of the pulping process with alkalisplitting is 32.7% ISO, which is above the result of the standardcooking process at 31.9% ISO. In spite of the low residual lignincontent, the yield of the pulping process with alkali splitting is anaccept 51.3% with absolutely dry wood. This is only a little less thanthe result of the standard cooking process, which has a yield of 52.0%accept with absolutely dry wood. “Accept” means the yield of fibrespassing through the slot sieve with an aperture size 0.15 mm afterpulping.

EXAMPLE 11

[0060] The NaOH was added in 4 equal doses of 25% each, wherein a firstportion was added at the beginning of the pulping process, a secondportion after 40 minutes (at about 140° C.), a third portion after 90minutes when the maximum temperature was reached, and a last portion of25% after 120 minutes, i.e. 30 minutes after the maximum temperature wasreached. The remaining conditions of Example 1 were left unchanged.

[0061] The cellulose pulped using four equal portions of NaOH shows avery low residual lignin content, even lower than the one obtained usingtwo portions of NaOH, as shown in Table 5. Yield and viscosity, i.e.also the strength characteristics, are at a very high level. This is aresult which is impossible to achieve with pulping processes where theentire alkali component is added at the beginning, or where the goal(cf. Ingruber) is to maintain a maximally high alkali level from thestart of the pulping process.

[0062] The evaluation of the tests of the present Example 11 has shownthat the addition of the at least one second portion of the alkalinecomponent results in particularly positive effects on delignificationand selectivity at a process temperature of 140° C. or more.

EXAMPLE 12

[0063] Spruce was pulped with a maximum pulping temperature of 175° C.,an overall percentage of chemicals of 27.5% with absolutely dry wood.The alkali ratio was adjusted to 60 to 40 of sulfite to alkali. FIG. 4shows how much of the alkali of the first portion—37.5% of all thealkali—was used up, which first portion is added at the beginning of thepulping process (conditions as in Example 1). The content of theremaining alkali is indicated in absolute percentages. The graphs thusshow that 37.5 NaOH was added at the beginning of the pulping process,while as early as 10 minutes later only about 25% NaOH is measurable.The content of NaOH is reduced to about 5% after 30 minutes andsignificantly rises only after 120 minutes when the second portion ofNaOH is added.

[0064] The amount of the residual alkali detectable in the aqueoussolution was determined by titration. A first titration to detect theremaining NaOH was carried out using hydrochloric acid directly (withoutBaCl₂). A more accurate titration was achieved by first neutralizing theresidual alkali with barium chloride (BaCl₂) before the titration wascarried out. The BaCl₂ also transforms the carbonate remaining in theaqueous solution, which has an effect on the pulp. The graphs show thatthe residual alkali titrated with or without BaCI₂ vary, however, onlyslightly in absolute values.

[0065] As early as 10 minutes after the heating has begun, ca. 30% ofthe initially applied first portion of the alkali is used up. After 30minutes of heating about 90% of the initially applied first portion ofalkali is used up. After 60 minutes of heating about 95% of theinitially applied alkali is used up. FIG. 4 thus shows with particularclarity how the method according to the present invention and thecellulose manufactured thereby differ from the recommendations of theprior art (according to Ingruber, in particular). TABLE 1 Effects ofmodifications in pulping of an alkaline sodium sulfite pulp with theaddition of anthraquinone Maximum pulping temperature: 175° C., pulpingduration: 150 minutes, raw material: pine overall kappa viscositytearing tear factor Test yield (%) number (mg/l) strength (km)* (cN)*standard 46.9 31.3 1131 11.1 111.4 modified 46.0 22.9 1204 11.1 124.2

[0066] TABLE 2 Effects of modifications in pulping of an alkaline sodiumsulfite pulp with the addition of anthraquinone Maximum pulpingtemperature: 175° C., pulping duration: 150 minutes, raw material:spruce overall kappa viscosity tearing tear factor Test yield (%) number(mg/l) strength (km)* (cN)* standard 52.7 35.4 1154 12.2 110.8 modified53.2 25.0 1245 12.2 117.9

[0067] TABLE 3 Effects of modifications in pulping of an alkaline sodiumsulfite pulp with the addition of anthraquinone and methanol (formodified ASAM); maximum pulping temperature: 175° C., pulping duration:150 minutes, raw material: spruce overall kappa viscosity tearing tearfactor Test yield (%) number (mg/l) strength (km)* (cN)* modified 46.921.4 1210 10.8 135.9 modified 47.7 16.4 1181 10.8 131.1 ASAM

[0068] TABLE 4 Effects of modifications in pulping of an alkaline sodiumsulfite pulp without the addition of anthraquinone Maximum pulpingtemperature: 175° C., pulping duration: 150 minutes, raw material:spruce Test kappa number standard 100.0 modified 92.8

[0069] TABLE 5 Effects of modifications in pulping of an alkaline sodiumsulfite pulp with the addition of anthraquinone Raw material: spruceoverall kappa viscosity tearing tear factor Test yield (%) number (mg/l)strength (km)* (cN)* modified: 50.1 23.7 1297 11.3 123.3 170° C., 210min modified 51.8 27.6 1341 11.3 115.9 165° C., 270 min modified 48.923.7 1191 11.1 127.2 4 portions NaOH

[0070] TABLE 7 Effect of alkali splitting in ASA pulping of spruce wood(27.5% overall percentage of chemicals, alkali ratio = 60/40, 150 min at175° C.) NaOH tearing degree of percentage kappa viscosity strength*tear factor* whiteness at outset accept [%] splitter [%] number [ml/g][km] [cN] end pH [% ISO]  100% 45.7 2.1 26.7 1125 11.8 118.1 11.4 25.1  75% 45.6 2.8 25.8 1160 11.4 119.3 11.3 26.4   50% 45.7 2.7 23.0 124911.7 126.4 11.4 29.3   25% 45.4 2.3 20.2 1228 11.4 137.2 11.4 32.6 12.5%45.5 2.3 21.4 1202 12.2 112.6 11.3 32.7   0% 45.7 2.0 23.4 1116 10.2121.2 11.3 30.7

[0071] TABLE 6 Effect of alkali splitting in ASA pulping of beech wood(27.5% overall percentage of chemicals, alkali ratio 50/50, 155° C.)alkali dosage at net. degree of tearing tear outset yield yield acceptsplitter kappa viscosity end whiteness strength factor No. [%] [%] [%][%] [%] number [ml/g] pH [% ISO] [km] [cN] 24 0 50.1 52.8 52.2 0.6 17.91149 12.3 34.1 8.6 88.2 25 12.5 48.5 51.2 50.7 0.5 18.1 1159 12.3 34.98.5 90.1 26 25 47.8 51.0 48.8 2.2 20.9 1207 12.1 32.1 9.1 83.0 27 37.547.1 50.2 47.6 2.7 20.9 1229 12.1 33.4 9.1 87.9 28 50 44.7 47.6 45.2 2.419.9 1207 12.3 35.3 8.6 88.2 29 75 44.5 47.3 45.8 1.5 18.4 1170 12.232.2 8.7 90.2 30 100 45.3 47.9 47.2 0.7 17.6 1157 12.2 30.7 8.3 86.6 3125 46.7 49.5 47.8 1.6 18.7 1174 11.9 33.9 8.8 87.3 32 37.5 45.3 47.847.1 0.7 16.7 1168 11.9 33.7 8.8 87.3 39 50 45.3 48.0 46.1 1.9 18.4 122312.0 34.4 8.3 91.6

[0072] TABLE 8 Effect of alkali splitting in ASA pulping of beech wood(27.5% overall percentage of chemicals, alkali ratio 60/40, 150 min,175° C.) NaOH tearing tear degree of percentage accept splitter kappaviscosity strength factor end whiteness at outset [%] [%] number [ml/g][km] [cN] pH [% ISO]  100% 42.9 4.0 31.3 1131 11.1 111.4 11.2 25.0   75%42.7 5.3 28.4 1181 11.2 123.0 11.8 23.7   50% 42.2 3.7 22.9 1204 11.1124.3 10.9 27.8   25% 42.5 3.7 22.0 1185 11.6 118.4 11.1 28.9 12.5% 43.83.1 26.2 1171 10.9 97.1 11.1 29.6   0% 44.6 3.8 28.0 1140 11.6 113.8 9.526.0

1. A method for delignifying lignocellulosic raw materials usingsulfites in the presence of an alkaline component, in particular sodiumhydroxide or sodium carbonate or a mixture thereof, in an aqueoussolution while applying high temperatures and pressures, characterizedin that at the beginning of a pulping process a first portion of thealkaline component is added to said aqueous solution and that at leastone second portion of the alkaline component is added to said aqueoussolution at the beginning of delignification or later.
 2. The method fordelignifying according to claim 1, characterized in that said aqueoussolution includes a quionone component in addition to the sulfites andthe alkaline component.
 3. The method for delignifying according toclaim 1, characterized in that said aqueous solution includes a sulfidecomponent in addition to the sulfites and the alkaline component as wellas, if applicable, a quionone component.
 4. The method for delignifyingaccording to at least one of the preceding claims, characterized in thatan alcohol, preferably methanol, is added to said aqueous solutionincluding the sulfite, the alkaline and, if applicable, the quinoneand/or sulfide components.
 5. The method according to claim 1,characterized in that said at least one second portion of the alkalinecomponent is added after the pH value of said aqueous solution hasfallen during heating, at least by an amount of pH 0.3, preferably by anamount of pH 0.5, more advantageously by an amount of pH 1.0, mostadvantageously by an amount of pH 1.5, each time vis-à-vis the initialpH value of the pulp.
 6. The method according to claim 1, characterizedin that at least 30%, preferably at least 90%, advantageously at least95% of said first portion of the alkaline component is used up duringpulping, before said at least one second portion of the alkalinecomponent is added.
 7. The method according to claim 1, characterized inthat said at least one second portion of the alkaline component is added10 minutes after the beginning of the heating process or later,preferably 30 minutes after the beginning of heating or later, moreadvantageously 60 minutes after the beginning of heating or later, mostadvantageously 90 minutes after the beginning of heating or later. 8.The method according to claim 1, characterized in that said at least onesecond alkaline portion is added at a temperature of at least 75° C.,preferably at least 110° C., more advantageously at least 140° C., mostadvantageously at least 175° C.
 9. The method according to claim 1,characterized in that said second portion of the alkaline component isadded at the end of the heating process when the maximum pulpingtemperature has been reached.
 10. The method for delignifying accordingto claim 1, characterized in that the pulping of said lignocellulosicraw material is carried out in said aqueous solution including thesulfite and the alkaline and, if applicable, the quinone component witha pulping duration of at least 90 minutes, preferably at least 120minutes, more advantageously at least 150 minutes, most advantageouslyat least 360 minutes.
 11. The method for delignifying according to claim1, characterized in that the pulping of said lignocellulosic rawmaterial is carried out is said aqueous solution including the sulfiteand the alkaline and, if applicable, the quinone component at a maximumpulping temperature and a pulping duration of at least 30 minutes,preferably between 60 minutes and 360 minutes, more advantageouslybetween 120 minutes and 180 minutes.
 12. The method for delignifyingaccording to claim 1, characterized in that the maximum pulpingtemperature is between 150° C. and 190° C., preferably between 160° C.and 180° C.
 13. The method for delignifing according to at least one ofthe preceding claims, characterized in that between about 15 wt. % and80 wt. % of the alkaline component is added as said first portion andbetween about 85 wt. % and about 20 wt. % of the alkaline component isadded as said second portion, preferably between about 75 wt. % andabout 30 wt. % of the alkaline component is added as said first portionand between about 25 wt. % and about 70 wt. % of the alkaline componentis added as said second portion, most advantageously between about 60wt. % and 40 wt. %, in particular, about 50 wt. % of the alkalinecomponent is added as said first portion, and between about 40 wt. % andabout 60 wt. %, preferably about 50 wt. % of the alkaline component isadded as said second portion.
 14. The method for delignifying accordingto claim 1, characterized in that the overall percentage of chemicals isat least about 18 wt. %, preferably between about 22 and about 45 wt. %,more advantageously between about 25 wt. % and about 35 wt. %, mostadvantageously between about 28 wt. % and about 32 wt. % with referenceto the absolutely dry weight of the raw material to be delignified. 15.The method for delignifying according to claim 1, characterized in thatfor pulping the lignocellulosic raw material said sulfite and saidalkaline component are adjusted in a ratio of between 80 to 20 and 40 to60, preferably between 70 to 30 and 50 to 50, more advantageously 60 to40.
 16. The method for delignifying according to claim 1, characterizedin that at least a third, preferably at least a third and a fourthportion of the alkaline component is added to said aqueous solution. 17.The method for delignifing according to claim 1, characterized in thatsaid raw material to be delignified is vaporized before said aqueoussolution including the sulfite, the alkaline component and, ifapplicable, a quinone component, is added.